Method of converting saturated sulfur compounds of a hydrocarbon cut containing few or no olefins

ABSTRACT

The invention relates to a method of converting the mercaptans contained in a hydrocarbon cut containing few or no olefins to heavier sulfur compounds. The method comprises a first stage of mixing the cut to be treated with hydrogen and possibly an olefin cut, then a second stage of reaction of the mixture from the first stage on a catalyst containing at least one group VIII metal. The invention also relates to the application of the method to treatment of a straight-run gasoline.

FIELD OF THE INVENTION

The invention relates to a method for sweetening and desulfurizing hydrocarbon fractions generally containing less than 1% by weight of olefins, and sulfur mainly in form of mercaptans. The method described in the present invention is particularly suited for treating hydrocarbon fractions resulting from atmospheric distillation such as, for example, gasoline, kerosine or light gas oil fractions.

These hydrocarbon fractions contain variable proportions of mercaptans which provide them with a corrosive and ill-smelling acid character. The solutions generally selected for treating these mercaptans consist in oxidizing the mercaptans to disulfides, then in separating the latter, which are heavier, generally by distillation.

The present invention provides a technical solution for sweetening the fractions to be treated by converting the mercaptans to sulfides, and possibly for desulfurizing the fractions to be treated by separating the sulfides thus formed.

The method consists in mixing the hydrocarbon fraction to be treated with, on the one hand, another hydrocarbon fraction containing unsaturated compounds and, on the other hand, with hydrogen, then in passing the mixture through a catalyst of metal sulfide type under optimized conditions so as to promote addition of the mercaptans on the unsaturated compounds in order to form sulfur compounds of sulfide type. The sulfur compounds thus converted have a boiling-point temperature that is higher than their initial temperature and they can be separated afterwards so as to recover a sulfur-depleted hydrocarbon fraction.

BACKGROUND OF THE INVENTION

Reduction of the mercaptan content in hydrocarbon fractions directly obtained from atmospheric distillation is generally carried out by means of a method referred to as “sweetening”.

Many sweetening methods do not involve a decrease in the global sulfur content, but only dimerization of the mercaptans to disulfides to meet the specifications. This mercaptan oxidation is generally carried out catalytically with oxygen as the oxidizing agent.

On the other hand, extractive sweetening methods allow to remove the sulfur from the feed. They consist in contacting the distillate to be treated with a basic aqueous solution (most often based on sodium hydroxide) which allows the mercaptans to be recovered. The basic solution is then regenerated by catalytic oxidation of the sodium mercaptides to disulfides.

Patent U.S. Pat. No. 3,574,093 describes the implementation of such a method for lighter gasolines and cuts (C3-C4). Many improvements have been carried out to the method, such as the use of aliphatic amines in small amounts in solutions based on sodium hydroxide (patents U.S. Pat. No. 2,546,345 and U.S. Pat. No. 2,853,431). The method however requires a very large volume of solution and many extraction stages, which limits its significance.

The use of metal hydroxides is not limited to liquid-liquid extraction. The teaching of patent U.S. Pat. No. 2,808,365 shows that an “alkaline solid” obtained by action of sodium hydroxide on calcium carbonate can be used as the catalyst for oxidation of mercaptans in the presence of epoxides in low proportions in the feed.

Other supports such as activated charcoal were impregnated with a basic solution to act as catalysts (patent GB-763,625).

Many oxidizing molecules are used in supported form to carry out oxidation of mercaptans. They however have the drawback of often leaving unwanted metal traces in the sweetened effluent.

Patent U.S. Pat. No. 2,255,394 discloses the use of copper in CuCi₂ form whereas sweetening is performed in the presence of oxygen by adding organometallic compounds of Et₂Ni type directly to the feed according to patent U.S. Pat. No. 3,053,756.

Catalysts based on CrO₃ or, more recently, on cobalt phthalocyanine are also used. The advantage of the method using cobalt phthalocyanine is that it can be carried out in supported phase for simple sweetening or in basic aqueous phase in its extractive variant.

The list is not exhaustive and other metals were tested for oxidizing mercaptans to disulfides.

Hydrocarbon fraction sweetening can also be carried out by addition reaction of the mercaptans on the diolefins.

These reactions are notably considered in the treatment of gasolines containing both mercaptans and unsaturated hydrocarbon compounds, such as FCC (Fluid Catalytic Cracking) gasolines for example, wherein unsaturated compounds, most often of olefinic nature, are widely present.

Patent US-2003/0,094,399 A1 describes a method using a distillation column wherein conversion of the mercaptans to sulfides is performed at the top of the column through contact with a hydrogenation catalyst involving a group VIII metal.

Patent FR-2,821,851 A1 also describes a method for weighting mercaptans by addition on the olefins by means of a catalyst comprising at least one group VIII element.

The same type of reaction is considered in patent U.S. Pat. No. 5,659,106A on an acid catalyst such as a sulfonated resin. In this case, the reaction is carried out in the absence of hydrogen.

Non-catalytic solutions are also presented in the literature.

For example, in patent U.S. Pat. No. 2,694,034, a sweetening method involving unsaturated compounds is developed. A mercaptan-rich saturated naphtha is treated by adding an olefinic compound (1 to 10% by weight) in the presence of a phenylene diamine type inhibitor, in a proportion of 0.0001% to 1% by weight. After a sufficient storage time, the mercaptan concentration of the mixture meets the specifications.

In conclusion, no solution is provided in the literature for converting mercaptans in order to sweeten or even to desulfurize the hydrocarbon fractions resulting from atmospheric distillation, by catalytic means other than oxidation of the mercaptans to disulfides.

Solutions based on the addition of mercaptans to unsaturated compounds are described only for hydrocarbon fractions containing large amounts of olefins such as catalytically or thermally cracked gasolines. The present invention proposes a simple solution for converting or even separating the saturated sulfur compounds present in the hydrocarbon fractions resulting from atmospheric distillation.

SUMMARY OF THE INVENTION

The method described in the present invention provides a solution for sweetening or even partly desulfurizing hydrocarbon fractions free of olefins or containing low unsaturated compound proportions. What is referred to as hydrocarbon fractions containing few or no olefins are fractions with less than 5% olefins, preferably less than 1% olefins.

The method described in the present invention is particularly suited for treatment of hydrocarbon fractions resulting from atmospheric distillation such as, for example, gasoline, kerosine or light gas oil fractions.

The method consists in mixing with the hydrocarbon fraction to be treated hydrogen and possibly another hydrocarbon fraction containing olefins, referred to as olefinic fraction. This olefinic fraction generally results from cracking methods such as FCC, steam cracking or a coking plant. The hydrogen can come from any source present in the refinery. The necessary hydrogen amounts are generally small enough not to require an additional dedicated hydrogen production plant. The resulting mixture must be such that, on the one hand, the H2/olefin molar ratio ranges between 0.03 and 2, preferably between 0.05 and 1, more preferably between 0.2 and 0.8, and that, on the other hand, the olefin/mercaptan molar ratio ranges between 5 and 5000, preferably between 10 and 1000, more preferably between 80 and 600, or even between 150 and 400.

The mixture meeting the aforementioned two conditions is then injected into a reactor containing a catalyst likely to react saturated sulfur compounds such as the mercaptans present. To be effective, the reaction has to be carried out in the presence of hydrogen.

According to a preferred embodiment of the invention, the non-olefinic feed treated is a gasoline fraction resulting from atmospheric distillation whose end boiling point is below 250° C., preferably below 220° C. But, in some cases still belonging to the field of the invention, the feed to be treated can itself contain a certain amount of olefins to which only a determined amount of hydrogen has to be added to reach the afore-mentioned H2/olefin ratio range between 0.03 and 2, preferably between 0.05 and 1.

In cases where a certain amount of olefins also has to be added to the feed to be treated, these olefins generally come from an olefinic gasoline such as, for example, a cracked gasoline from a catalytic or thermal cracking plant.

The method of converting the mercaptans contained in a hydrocarbon feed containing less than 1% olefins according to the invention can comprise an additional stage of separation of the sulfur compounds formed during the reaction stage, so as to produce an effluent containing less than 50% by weight of the sulfur compounds present in the feed, and a cut containing the major part of the sulfur compounds.

The method according to the invention thus allows mercaptans to be converted to heavier sulfur compounds with a conversion rate of generally at least 50% by weight.

DETAILED DESCRIPTION

The invention can be defined as a method for treating a hydrocarbon feed, generally a gasoline containing few or no olefins, generally less than 5% olefins, typically less than 1% olefins, so as to convert and possibly to eliminate the sulfur compounds it contains, notably the mercaptans. The method according to the invention comprises at least 2 stages:

a first stage referred to as mixing stage, which consists in mixing the mercaptan-containing hydrocarbon feed to be treated with a certain amount of hydrogen and possibly with an olefinic fraction,

a second stage referred to as reaction stage, which consists in reacting the mixture resulting from the first stage on a catalyst comprising at least one group VIII metal so as to convert the mercaptans to sulfides. This reaction is referred to hereafter as conversion of mercaptans to sulfides.

The hydrocarbon feed to be treated generally contains less than 1% by weight of olefins and more than 50 ppm by weight of sulfur, mainly in form of mercaptans.

The present invention applies more particularly to the treatment of gasoline cuts directly resulting from atmospheric distillation, which are generally practically free of olefins and rich in saturated sulfur compounds. It can however be applied to other feeds such as distillates also containing few or no olefins.

What is referred to as saturated sulfur compounds are the sulfur compounds belonging to the mercaptan or sulfide family.

The boiling temperatures of the feed to be treated are below 350° C., preferably below 250° C., which generally corresponds to a gasoline cut.

The olefinic fractions present in the refinery generally come from cracking plants such as catalytic, thermal or steam cracking plants. The boiling temperatures of this olefinic fraction are generally below 250° C.

However, hydrocarbon fractions containing olefins with 4, 5 or 6 carbon atoms are preferably used in order to improve the yield of the sulfur compound weighting reaction.

The hydrogen also comes in most cases from the refinery. A hydrogen practically free of H₂S is used because this component can react with the olefins and form unwanted sulfur compounds. The hydrogen can come from either a specific hydrogen production plant or from the gasoline catalytic reforming plant for example.

The reaction stage consists in passing the mixture obtained at the end of the mixing stage through a fixed-bed catalyst under optimized operating conditions.

The catalyst used in the reactor is a catalyst comprising at least one group VIII metal deposited on an inert support based on a porous metal oxide. Preferably, the support consists of alumina, silica, titanium oxide, or it contains at least 50% alumina.

A group VIb metal can also be associated with the group VIII metal to form a bimetallic catalyst.

The proportion of group VIII metal in oxide form ranges between 1% and 30% by weight. The proportion of group VIb metal in oxide form ranges between 0% and 20% by weight.

Nickel-based catalysts or catalysts based on mixed nickel and molybdenum or tungsten oxide are preferably used.

Typically, the catalyst used in the reaction stage contains between 1% and 30% by weight of NiO and between 0% and 20% by weight of MoO₃.

Prior to injecting the feeds to be treated, the catalyst can first be subjected to a sulfurization stage in order to convert the metal oxides to sulfides.

Sulfurization is carried out in the presence of H₂S, either injected directly in admixture with hydrogen, or generated in situ in the reactor by hydrogenolysis of a sulfur compound, so that the sulfurization rate of the catalyst metals is above 50%, preferably above 90%.

The temperature of the reactor generally ranges between 100° C. and 250° C., preferably between 140° C. and 200° C.

The reactor is operated at a pressure ranging between 0.5 MPa and 5 MPa, preferably between 1 MPa and 3 MPa, and at a space velocity ranging between 1 h⁻¹ and 10 h⁻¹, preferably 1.5 h⁻¹ and 8 h⁻¹.

The hydrogen flow rate is generally adjusted in order to obtain a hydrogen/olefin molar ratio ranging between 0.03 and 2, preferably between 0.05 and 1.

Under such conditions, surprisingly, the saturated sulfur compounds present in the feed are converted to saturated sulfur compounds of higher boiling point temperature.

The saturated compounds belong to the families consisting of mercaptans, sulfides and CS₂.

The conversion is measured by the conversion rate of the mercaptans to heavier compounds, i.e. having a higher boiling point temperature.

Furthermore, the olefin/hydrogen molar ratio is optimized in order to limit deactivation of the catalyst by the carbon deposit due to the olefinic compounds.

In the absence of hydrogen, the catalyst undergoes a high deactivation and the mercaptan conversion rate is markedly decreased.

At the end of the reaction stage, the effluent is depleted in light saturated sulfur compounds and more particularly in mercaptans.

The conversion rate of mercaptans containing 1 to 4 carbon atoms generally ranges between 50% and 100%. The fraction thus produced is therefore sweetened in the sense known to the man skilled in the art.

A third stage can optionally be carried out if it is desired to lower the sulfur content of the feed to be treated.

This third stage, referred to as separation stage, consists in separating the sulfur compounds formed during the reaction stage from the hydrocarbon effluent of said reaction stage.

The separation stage can consist of any method capable of achieving this separation.

However, a physical separation method based on the boiling point temperatures of the compounds to be separated such as, for example, a simple flash or distillation in a distillation column is preferably used.

In this case, the light fraction recovered at the top of the column contains the major part of the saturated hydrocarbon fraction and it is depleted in sulfur compounds and in mercaptans.

The heavy fraction collected at the bottom of the column concentrates the sulfur compounds formed during the reaction stage. This fraction can be treated in a hydrodesulfurization plant to extract the sulfur therefrom. Implementation of the separation stage after the reaction stage thus allows to desulfurize the hydrocarbon feed to be treated without requiring a conventional hydrodesulfurization method for at least part of the hydrocarbon effluent.

EXAMPLES

A series of tests was performed in a pilot plant with 100 cm³ catalyst.

The catalyst used contains nickel and molybdenum on an alumina support (catalyst marketed under reference HR845 by the Axens Company). Prior to injecting the feed, the catalyst is sulfurized by a H₂+H₂S mixture at 350° C. During the tests, the temperature, the pressure and the space velocity are maintained constant respectively at 180° C., 2.5 MPa and 4 h⁻¹ for all the tests carried out.

Example 1 According to the Prior Art

In this example, a gasoline A resulting from atmospheric distillation of a crude oil is injected into a reactor in the absence of hydrogen.

At the reactor outlet, effluent B 1 is separated by distillation into two fractions with a cut point corresponding to a temperature of 100° C. The two fractions obtained are denoted by LCN1 (light fraction) and HCN1 (heavy fraction). The characteristics of the various gasolines are given in Table 1. TABLE 1 Characteristics Gasoline A Effluent B1 LCN1 HCN1 T_(5%)-T_(95%) 25-125 25-125 25-100 100-125 Yield per fraction (%) 100 100 74.6 25.4 15/4 density 0.702 0.705 0.68 0.763 Total sulfur (ppm) 300 300 135 783 Mercaptan sulfur 160 129 135 111 (ppm) Olefin/RSH (mol/mol) 2.4 — — — H₂/Olefin (mol/mol) 0 — — — Gas chromatography (% wt) Paraffins 67.4 67.4 74.6 47.4 Olefins 0.1 0.1 0.1 0.0 Naphthenes 26.2 26.2 23.4 34.0 Aromatics 6.3 6.3 1.9 18.6

The presence of the catalyst allows the mercaptan content to be reduced by 20%.

However, the mercaptan content of cut LCN1 remains high, and the mercaptan distribution between cuts LCN1 and HCN1 shows that the conversion observed equally affects the light mercaptans and the heavy mercaptans with at least five carbon atoms. Furthermore, the organic sulfur content remains unchanged during treatment on the catalyst.

The absence of hydrogen and a very low unsaturated compound content in the feed do not allow to obtain a notably desulfurized light cut. However, the sulfur of this fraction is present exclusively in mercaptan form, so that the non-mercaptan sulfur is consequently concentrated in the heavy fraction.

Example 2 According to the Invention

Gasoline A is mixed with an olefinic cracked gasoline C from a catalytic cracking plant prior to being injected into the reactor in the presence of hydrogen in a proportion of 5 litres hydrogen per litre feed. Gasoline C represents 10% by weight of the mixture referred to as gasoline D1. Gasoline D1 mixed with the hydrogen is injected on the catalyst.

At the reactor outlet, effluent B2 is separated by distillation into two fractions with a cut point corresponding to a temperature of 100° C. The two fractions obtained are denoted by LCN2 (light fraction) and HCN2 (heavy fraction). Table 2 shows the characteristics of gasolines A, C and D1. The characteristics of the various cuts are given in Table 3. TABLE 2 Characteristics Gasoline A Gasoline C Gasoline D1 T_(5%)-T_(95%) 25-125 25-215 25-215 15/4 density 0.702 0.746 0.707 Total sulfur (ppm) 300 177 279 Mercaptan sulfur (ppm) 160 8 151 Olefin/RSH (mol/mol) 2.4 16,430 91 Gas chromatography (% wt) Paraffins 67.4 28.9 63.5 Olefins 0.1 34.5 3.6 Naphthenes 26.2 7.6 24.4 Aromatics 6.3 29 8.5

TABLE 3 Characteristics Gasoline D1 Effluent B2 LCN2 HCN2 T_(5%)-T_(95%) 25-215 25-215 25-100 100-215 Fraction/Gasoline D1 100 100 71.5 28.5 (% wt) 15/4 density 0.707 0.708 0.683 0.779 Total sulfur (ppm) 279 279 34 893 Mercaptan sulfur 151 64.5 34 141 (ppm) Olefin/RSH (mol/mol) 91 — — — H₂/Olefin (mol/mol) 0.71 — — — Gas chromatography (% wt) Paraffins 63.5 63.6 72.3 43 Olefins 3.6 3.5 3.2 4.1 Naphthenes 24.4 24.4 22.7 28.4 Aromatics 8.5 8.5 1.9 24.6

The simultaneous presence of olefins and of hydrogen in mixture D1 allows the initial mercaptan content to be reduced by 57%. The conversion is three times the conversion observed in example 1. Furthermore, the mercaptan distribution between cuts LCN 2 and HCN2 is widely modified, the light mercaptans contained in cut LCN2 are preferentially converted.

Example 3 According to the Invention

Gasoline A is mixed with cracked gasoline C prior to being injected into the reactor in the presence of hydrogen in a proportion of 5 litres hydrogen per litre feed. Gasoline C represents 20% by weight of the mixture denoted by gasoline D2.

At the reactor outlet, effluent B3 is separated by distillation into two fractions with a cut point corresponding to a temperature of 100° C. The two fractions obtained are denoted by LCN3 (light fraction) and HCN3 (heavy fraction). Table 4 shows the characteristics of gasolines A, C and D2. The characteristics of the various cuts are given in Table 5. TABLE 4 Characteristics Gasoline A Gasoline C Gasoline D2 T_(5%)-T_(95%) 25-125 25-215 25-215 15/4 density 0.702 0.746 0.715 Total sulfur (ppm) 300 177 270 Mercaptan sulfur 160 8 134 (ppm) Olefin/RSH (mol/mol) 2.4 16,430 196 Gas chromatography (% wt) Paraffins 67.4 28.9 59.8 Olefins 0.1 34.5 6.9 Naphthenes 26.2 7.6 22.5 Aromatics 6.3 29 10.8

TABLE 5 Characteristics Gasoline D2 Effluent B3 LCN3 HCN3 T_(5%)-T_(95%) 25-215 25-215 25-100 100-215 Fraction/Gasoline D2 100 100 68.5 31.5 (% wt) 15/4 density 0.715 0.717 0.685 0.788 Total sulfur (ppm) 270 270 32 789 Mercatan sulfur (ppm) 134 71 32 156 Olefin/RSH (mol/mol) 196 — — — H₂/Olefin (mol/mol) 0.37 — — — Gas chromatography (% wt) Paraffins 59.8 59.9 69.7 39.4 Olefins 6.9 6.8 6.6 7.2 Naphthenes 22.5 22.5 21.9 23.8 Aromatics 10.8 10.8 1.8 29.6

The increase in the amount of olefins injected does not significantly improve the performances, the mercaptan conversion stagnates around 50%. The H₂/olefin ratio is decreased, which favours catalyst deactivation problems.

Example 4 According to the Invention

Gasoline A is mixed with cracked gasoline C prior to being injected into the reactor in the presence of hydrogen in a proportion of 10 litres hydrogen per litre feed.

Gasoline C represents 10% of the mixture that is denoted by gasoline D3.

At the reactor outlet, effluent B4 is separated by distillation into two fractions with a cut point corresponding to a temperature of 100° C. The two fractions obtained are denoted by LCN4 (light fraction) and HCN4 (heavy fraction). Table 6 gives the characteristics of the various gasolines. TABLE 6 Characteristics Gasoline D3 Effluent B4 LCN4 HCN4 T_(5%)-T_(95%) 25-215 25-215 25-100 100-215 Fraction/Gasoline 100 100 71.6 28.4 D1 (% wt) 15/4 density 0.707 0.704 0.682 0.78 Total sulfur (ppm) 279 279 32 901 Mercaptan sulfur 151 63 32 141 (ppm) Olefin/RSH 91 — — — (mol/mol) H₂/Olefin (mol/mol) 1.43 — — — Gas chromatography (% wt) Paraffins 63.5 63.8 72.4 43.4 Olefins 3.6 3.2 3 3.8 Naphthenes 24.4 24.4 22.7 28.4 Aromatics 8.5 8.5 1.9 24.6

The presence of unsaturates and the presence of hydrogen are two factors favouring mercaptan conversion. However, the H₂/olefin ratio also has to be optimized to limit catalyst deactivation, hydrogen consumption and olefin saturation.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

In the foregoing and in the examples, all temperatures are set forth uncorrected in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.

The entire disclosures of all applications, patents and publications, cited herein and of corresponding French application No. 04/12.206, filed Nov. 17, 2004 are incorporated by reference herein.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. 

1) A method of converting mercaptans contained in a hydrocarbon feed containing less than 5% by weight of olefins, comprising at least two stages: a first stage of mixing the feed to be treated with an amount of hydrogen, and optionally a stream containing the resulting mixture having a H₂/olefin molar ratio ranging between 0.03 and 2, and an olefin/mercaptan molar ratio ranging between 5 and 5000, and a reaction stage comprising reacting of the mixture formed at the end of the first stage on a supported catalyst containing at least one group VIII metal, so as to convert mercaptans to heavier sulfur compounds with a conversion rate of at least 50%. 2) A mercaptan conversion method as claimed in claim 1, wherein said hydrocarbon feed contains less than 1% by weight of olefins. 3) A mercaptan conversion method as claimed in claim 1, wherein the reaction stage is carried out at a temperature ranging between 100° C. and 250° C. 4) A mercaptan conversion method as claimed in claim 1, wherein the reaction stage is carried out at a pressure ranging between 0.5 MPa and 5 MPa. 5) A mercaptan conversion method as claimed in claim 1, wherein the reaction stage is carried out at a space velocity ranging between 1 h⁻¹ and 10 h⁻¹. 6) A mercaptan conversion method as claimed in claim 1, wherein the feed to be treated is a straight-run gasoline. 7) A mercaptan conversion method as claimed in claim 1 comprising adding to the feed to be treated an amount of catalytically cracked gasoline. 8) A mercaptan conversion method as claimed in further comprising withdrawing mercaptan-depleted effluent from the reaction stage and separating said effluent from the sulfur compounds formed during said reaction stage so as to produce an effluent containing less than 50% sulfur compounds present in the feed, and a cut containing the major part of the sulfur compounds. 9) A mercaptan conversion method as claimed in claim 8, wherein said separating is conducted by flashing or by distillation. 10) A mercaptan conversion method as claimed in claim 1, wherein the catalyst used in the reaction stage also contains at least one group VIb metal. 11) A mercaptan conversion method as claimed in claim 1, wherein the catalyst used in the reaction stage contains between 1% and 30% by weight of group VIII metal in oxide form, and 0% to 20% by weight of group VIb metal in oxide form. 12) A mercaptan conversion method as claimed in claim 1, wherein the catalyst used in the reaction stage contains between 1% and 30% by weight of NiO, and between 0% to 20% by weight of MoO₃. 13) A mercaptan conversion method as claimed in claim 1, wherein the catalyst used in the reaction stage is first subjected to a sulfurization stage so that the proportion of metal present in sulfide form is above 50%. 14) A method according to claim 1 wherein said resulting mixture has an H₂ olefin molar ration between 0.05 and
 1. 15) A method according to claim 1 wherein said olefin/mercaptan molar ration is between 10 and
 1000. 16) A method according to claim 14 wherein said olefin/mercaptan molar ration is between 10 and
 1000. 17. A method as claimed in claim 1 wherein the reaction s conducted at a temperature of 140°-240° C. and a pressure of 1 MPa to 3 MPa, and a space velocity of between 1.5 h⁻¹ and 8 h⁻¹. 18) A method according to claim 11 wherein the catalyst contains said group VIb metal in oxide form. 19) A method according to claim 12 wherein the catalyst contains MoO₃. 20) A method according to claim 13 wherein the proportion of metal present in sulfide form is above 90%. 